1. Field of the Invention
The present invention relates to a condenser-discharge type ignition device for an internal combustion engine.
2. Description of the Related Art
FIG. 6 illustrates a conventional ignition device for an internal combustion engine. Referring to FIG. 6, a drive circuit 3 is connected to an oscillating circuit 2. A transistor 4 serving as a first switching device is connected to the drive circuit 3. An end portion of the primary coil of a transformer is connected to the collector of the transistor 4. The above-described oscillating circuit 2, the drive circuit 3, the transistor 4 and the transformer 5 constitute a DC-DC converter. A rectifying diode 6 is connected to the secondary coil of the transformer 5. A capacitor 8 is connected between the cathode of the rectifying diode 6 and ground. An end portion of the primary coil of an ignition coil 10 is connected to a junction between the cathode of the rectifying diode 6 and the capacitor 8. Furthermore, a thyristor 11 serving as a second switching device is connected to a portion between another end portion of the primary coil of the ignition coil 10 and the ground. Furthermore, an ignition plug 15 is connected to the secondary coil of the ignition coil 10.
A trigger circuit 17 for generating a trigger signal in response to ignition signal S.sub.2 is connected to an ignition signal generating circuit 16 which generates the ignition signal S.sub.2 in synchronization with the internal combustion engine. The trigger circuit 17 is connected to the gate terminal of the thyristor 11. A battery 1 is connected to the oscillating circuit 2, the drive circuit 3, another terminal of the primary coil of the transformer 5, the ignition signal generating circuit 16 and the trigger circuit 17.
Then, the operation of the above-described conventional ignition device will now be described. As shown in FIG. 7, the drive circuit 3 transmits a drive signal to the transistor 4 in response to signal S.sub.1 transmitted from the oscillating circuit 2. As a result, the transistor 4 is driven so that the transformer 5 is electrically turned on/off. At this time, primary current I.sub.1 generated in the primary coil of the transformer 5 is 7 converted into secondary current I.sub.2 of the transformer 5 so that the capacitor 8 is charged via the diode 6. The capacitor 8 is charged to a voltage level of V.sub.1 as shown in FIG. 7.
The ignition signal generating circuit 16 generates the ignition signal S.sub.2 in synchronization with the ignition timing of the internal combustion engine. In response to the ignition signal S.sub.2 thus-generated, the trigger circuit 17 transmits the trigger signal to the gate terminal of the thyristor 11. When the thyristor 11 is thus-triggered, the charge, which has been stored in the capacitor 8, is discharged via the primary coil of the ignition coil 10 and the thyristor 11. At this time, discharge current I.sub.4 as shown in FIG. 7 is introduced into the thyristor 11 from the capacitor 8. As a result, high voltage is generated in the secondary coil of the ignition coil 10 so that the ignition plug 15 is ignited.
However, if the secondary current I.sub.2 flows in the secondary coil of the transformer 5 when the discharge current I.sub.4 is introduced into the thyristor 11 from the capacitor 8, the secondary current I.sub.2 of the transformer 5 is as well introduced into the thyristor 11 via the diode 6 and the primary coil of the ignition coil 10. That is, total current I.sub.5 passing through the thyristor 11 is the sum of the discharge current I.sub.4 from the capacitor 8 and the secondary current I.sub.2 of the transformer 5.
Therefore, although the total current I.sub.5 does not exceed a predetermined value in a case where the timing at which the thyristor 11 is electrically turned on is, as shown in FIG. 7, different from the timing at which the transformer 5 generates the secondary current I.sub.2 as at time t.sub.1 and t.sub.3, the total current I.sub.5 is excessively enlarged in a case where the timing at which the thyristor 11 is electrically turned on coincides with the timing at which the transformer 5 generates the secondary current I.sub.2 as at time t.sub.2. As a result, there arises a problem that the overall apparatus size and the cost cannot be reduced because the electric current capacity of the thyristor 11 cannot be reduced in order to protect the thyristor 11 from breakage.
FIG. 8 illustrates the structure of another conventional ignition device for an internal combustion engine. The ignition device of this type further comprises, in addition to the elements of the ignition device shown in FIG. 6, a second rectifying diode 7 connected to the secondary coil of the transformer 5. Furthermore, a second capacitor 9 is connected to a portion between the cathode of the rectifying diode 7 and the ground. In addition, a pulsating current preventing diode 12 is, in parallel, connected to the primary coil of the ignition coil 10. An inductor 13 for maintaining discharge time is connected to a portion between the cathodes of the diodes 7 and 12. Furthermore, a diode 14 for maintaining the discharge time is connected to a portion between the cathode of the diode 7 and the anode of the diode 12. The remaining elements are the same as those for the ignition device shown in FIG. 6.
The operation of the ignition device of this type will now be described. Similarly to the ignition device shown in FIG. 6, the second capacitor 9 is also charged via the diode 7 at the same time at which the capacitor 8 is charged. In this case, the capacitor 9 is charged with a voltage level of V.sub.2 as shown in FIG. 9.
The ignition signal generating circuit 16 generates ignition signal S.sub.2 at the ignition timing of the internal combustion engine. In response to the ignition signal S.sub.2, a trigger signal is transmitted from the trigger circuit 17 to the gate terminal of the thyristor 11. When the thyristor 11 is triggered, the charge, which has been stored in the capacitor 8, is discharged via the primary coil of the ignition coil 10 and the thyristor 11. On the other hand, the charge stored in the capacitor 9 is discharged via the inductor 13, the primary coil of the ignition coil 10 and the thyristor 11. As a result, output voltage V.sub.3 and output current I.sub.3 as shown in FIG. 9 are generated in the secondary coil of the ignition coil 10 so that the ignition plug 15 is ignited.
At this time, a discharge maintaining current flows from the primary coil of the ignition coil 10 via the diode 14 and the inductor 13. As a result, the discharge made by the ignition plug 15 is maintained for .DELTA.t.sub.1.
However, if the capacitors 8 and 9 are charged by energy supplied from the DC-DC converter during the above-described time .DELTA.t.sub.1 in which the discharge is maintained as at time t.sub.4, the above-described discharge maintaining current is stopped, causing a problem to arise in that the discharge is undesirably temporarily stopped.
Another problem takes place in that the size of the DC-DC converter cannot be reduced because both of the above-described conventional ignition devices similarly employ the transformer 5 in the DC-DC converter. In addition, the charging efficiency of each of the capacitors 8 and 9 deteriorates due to the conversion efficiency of the primary and the secondary sides of the transformer 5.